This invention relates to friction liners for dry friction devices, the liner being of a flat, annular general form and being of the type comprising a flat annular body of friction material which is bounded by an inner cylindrical surface and an outer cylindrical surface. The invention also relates to a method of making such a friction liner, and to a rotatable clutch friction wheel for working in dry friction and having at least one friction liner of the kind defined above.
Numerous examples of such annular friction liners are known, which are in the form of a flat annular ring, and which are used for equipping a clutch friction wheel or clutch disc, or a disc brake, in either case using dry friction in operation. Each friction liner, in the form of a flat annular ring, is made in the form of a workpiece, that is to say an embryonic or partly finished liner, which undergoes a final curing operation in order to obtain polymerisation of some of its constituents, and this curing operation may in appropriate cases be followed by a post-curing operation.
The flat annular workpiece may, in accordance with various known methods, consist for example of threads which are impregnated with a thermosetting resin, and which are cut to variable lengths of between 5 and 30 mm, the workpiece being preformed in a mould.
Alternatively, it may be based on threads which are impregnated and braided or plaited, the length of the braid or plait corresponding to the outer diameter of the workpiece.
The workpiece may consist of threads which are impregnated and woven, so as to obtain a band of fabric the length of which corresponds to the outer diameter of the workpiece.
Again, the workpiece may be made from impregnated threads which are wound or rolled on a mandrel to the internal diameter of the workpiece, the tube thus obtained by winding being subsequently cut off, or sliced, into lengths, so as to produce a corresponding number of flat annular workpieces to be subsequently completed by curing.
Finally, the workpiece may be made from impregnated threads which are wound or rolled by superimposition of lobes corresponding to the outer diameter of the workpiece.
All the above methods of making the unfinished liners (or workpieces), as described above, have as their main drawback the fact that they result in the production of annular friction liners which have low centrifugal strength, especially under hot conditions, that is to say they have poor resistance to rupture under the effects of centrifugal force; this effect is more marked the higher the working temperature.
Rupture under centrifugal stress is in general due to the fact that the maximum tangential stress in the region of the cylindrical inner surface of the crown-shaped liner exceeds the tensile rupture stress of the friction material of which it is made.
An object of the invention is to provide an improved friction liner of the general type mentioned above, in which the centrifugal strength of the liner is improved and its selling cost is reduced.
According to the invention in a first aspect, a friction liner for a dry friction device, the liner being of a flat annular general form and being of the type comprising a flat annular body of friction material bounded by a cylindrical inner surface and a cylindrical outer surface, is characterised in that it includes at least one annular reinforcing zone.
According to a preferred feature of the invention, the annular reinforcing zone is disposed close to the inner cylindrical surface of the liner.
According to another preferred feature of the invention, the liner has several annular reinforcing zones arranged substantially concentrically. Preferably, the annular reinforcing zones are arranged substantially concentrically, the innermost reinforcing zone being in the vicinity of the cylindrical inner surface of the liner. In some preferred embodiments, the concentric annular reinforcing zones are distributed over the whole of the liner between the cylindrical inner surface and the outer surface of the latter. In some embodiments, the annular reinforcing zones are made in a material of the same density. In other embodiments the annular reinforcing zones are made in materials of different densities.
In preferred embodiments having the last mentioned arrangement and with the annular reinforcing zones being disposed substantially concentrically, the innermost one of these zones being in the vicinity of the cylindrical inner surface of the liner, the annular reinforcing zones are made in materials of different densities, with these densities decreasing in a radial direction away from the vicinity of the cylindrical inner surface of the liner.
According to a further preferred feature of the invention, each annular reinforcing zone is a reinforcing ring constituted by a substantially annular cylindrical layer inserted in the flat annular body.
According to yet another preferred feature of the invention, each annular reinforcing zone comprises an organic, mineral and/or metallic reinforcing material.
According to a still further preferred feature of the invention, each annular reinforcing zone comprises a reinforcing material in the form of a continuous thread, a discontinuous thread, fibres, woven material, a braid or plait, or a tape.
Preferably, the liner includes a friction material consisting of a mat of fibres impregnated with a thermosetting resin. In that case, preferably, the said fibres have a length in the range between 40 mm and 120 mm.
Preferably, the fibres are selected among the group of fibres consisting of cotton, viscose, linen, polyacrylonitryl, preoxidised polyacrylonitryl, para-aramide, meta-aramide, and mineral fibres. Examples of such mineral fibres are E-C-R glass, rockwool, and ceramics.
Glass is preferably incorporated in the mat, which may be in the form of random fibres, and/or roving, and/or a textured or bulked mat. The diameters of the glass fibres are preferaby in the range between 6 and 21 micrometers. The glass fibres are preferably subjected to adhesive bonding, impregnation with phenolic resin, and/or impregnation with rubber.
In preferred embodiments, powder fillers are incorporated in the mat, and comprise, wholly or partly, any one or more of the following constituents: copper, rockwool, powdered nitryl rubber, carbon black, hexamethylene tetramine, gilsonite, litharge, melamine resin, phenolic resin, sulphur, cardolite, zirconium silicate, iron sulphate, alumina, latex, powdered scrap clutch friction liners, friction liner rectification powders, zinc oxide.
According to the invention in a second aspect, a method of making a friction liner for a dry friction device, the liner being of generally annular flat form and being of the type comprising an annular flat body of friction material, wherein a workpiece consisting of the unfinished annular flat body is made by winding, on a mandrel, a band of predetermined length of a mat of friction material, is characterised in that, during the winding step, at least one length of a band of reinforcing material, having a length sufficient to constitute at least one annular reinforcing zone, is inserted so as to obtain subsequently a friction liner according to the invention in its first aspect.
According to various preferred but optional features, which may be incorporated in the method of the invention either singly, or if desired, in such combinations as are technically possible:
(a) a mixture of fibres of the same nature or of different natures as defined above is made in a mixer;
(b) the mixture is carded so as to form a carded web;
(c) the carded web is napped;
(d) glass is incorporated in the carded web at the instant of napping, this being preferably done between steps (b) and (c) above, with powder fillers being sprinkled on the carded web; and carding is preferably carried out using a wool type card;
(e) the mat is sized to a thickness of between 3 and 10 mm between two heated rollers, the temperature being preferably between 50 and 100xc2x0 C., and the pressure being preferably between 1 and 10 bars;
(f) the mat is cut into bands, preferably along its length, the width of these bands being preferably between 8 and 25 mm;
(g) the bands thus cut off are welded together using heated plates, preferably at a temperature between 50 and 150xc2x0 C.; these welded bands are preferably overlapped by an amount in the range between 3 and 20 mm;
(h) the welded band is wound on a roller, with reinforcing material being introduced so as to form an annular ring.
In order to form a device with a coating of friction material, the invention provides the following method:
(i) an annular ring, or in another version a plurality of sectors constituting an annular ring, is cut from the mat made as described above;
(j) the annular ring or plurality of sectors constituting a ring is put in the bottom of a mould;
(k) curing under pressure is carried out in the mould.
According to the invention in a third aspect, a rotatable clutch friction wheel, or friction disc, for working under dry friction, includes at least one friction liner in accordance with the invention, which is preferably made in accordance with the method of the invention.
The invention enables a friction liner to be obtained which has good friction characteristics and high resistance to wear, besides its high resistance to centrifugal force.
The invention has the advantage of enabling friction liners to be obtained in which their characteristics of friction and wear resistance on the one hand, and mechanical strength against centrifugal force on the other, can be selected and controlled separately.
Further features and advantages of the invention will appear more clearly on a reading of the following description of some preferred embodiments of the invention, given by way of non-limiting example only and with reference to the accompanying drawings.